At least 26 people have been killed after a gunman opened fire at a church during Sunday services, officials say.

The attack happened at the First Baptist Church in Sutherland Springs, a small town in Wilson County, Texas.

The gunman, who is reported to have been killed in the aftermath, entered the church and opened fire at around 11:30 local time (17:30 GMT).

Governor Greg Abbott confirmed the death toll and said it was the worst mass shooting in the history of Texas.

"This will be a long, suffering mourning for those in pain," he said at a news conference on Sunday.

Texas Department of Public Safety regional director Freeman Martin said the ages of the victims ranged from 5 to 72 years old. Authorities said at least 20 people were also injured and taken to hospital.

Mr Martin described the suspect as a young white male in his early 20s, dressed in all black and wearing a ballistic vest as well as tactical-type gear. He was armed with an assault-style rifle.

He began shooting outside the church before he entered and opened fire, Mr Martin said.

A local citizen "grabbed his rifle and began shooting" at the suspect, before the gunman fled in a vehicle.

The citizen pursued the suspect, who drove off the road and crashed his car at the Guadalupe County line. Police found the suspect dead in his car, but it is unclear if he died from a self-inflicted gunshot wound or from returned fire from the citizen, Mr Martin added.

The gunman has been identified as Devin P Kelley, 26, according to US media reports. Police have not confirmed the suspect's identity.

The First Baptist Church's pastor, Frank Pomeroy, has told ABC News that his 14-year-old daughter, Annabelle, was killed in the shooting.

Mr Pomeroy, who was out of town in Oklahoma at the time, described her as "one very beautiful, special child" in a phone call to the television outlet.

At least 10 victims, including four children, were being treated at the University Health System in nearby San Antonio, the hospital said in a tweet.

Sheriff Joe Tackitt said authorities do not know the names of any of the victims yet as they continue to work the crime scene.

Officials said 23 people were found dead inside of the church while two people were fatally shot outside. Another individual died in hospital, authorities said.